The present invention relates to cathode ray tube displays.
Cathode ray tubes (CRT) produce a two dimensional visual display by raster scanning an electron beam over a phosphorescent surface (screen). The phosphor emits light of varying intensity in accordance with the number of electrons striking the screen. The electron beam is scanned across successive horizontal lines to illuminate the entire screen on a point-by-point basis, and trace a two dimensional picture on the face of the tube. While the electron beam illuminates only one spot at a given instant, the scanning is effected at a rate beyond the maximum rate to which the human eye can respond. Thus, the entire screen appears to the human observer as being fluorescent, and the overall picture is perceived by the observer.
More specifically, a composite video signal, including video and synchronizing information in accordance with standards set out in, for example, Electronic Industries Association (EIA) standards RS-170 and RS-343, is amplified, and applied to the electron gun of the CRT. Potentials are applied to the CRT anodes (A1 and A2) to control the energy (acceleration) in the beam, and hence the brightness of the displayed image. Focusing of the beam is accomplished by applying voltages to grids G1 and G2. In high resolution displays provisions are usually made to modulate the focus potential with a parabolic waveform of suitable amplitude and phase so that sharp beam focus is maintained over the entire screen.
The scanning is effected by deflecting the electron beam with an electrostatic or magnetic field. Signals with generally sawtooth waveforms are applied to either respective vertical and horizontal plates, or respective sets of deflection coils, disposed so that electron beam passes through the electrostatic or magnetic fields generated thereby. Deflection coils are generally disposed in a yoke set around the neck of the CRT, and include two pairs of coils, each mounted at right angles to the other. The two coils of each pair are generally connected in series and mounted on opposite sides of the neck of the tube.
It should be appreciated, that the particular deflection signals required to provide a full raster scan of the CRT, is a function of the structure and geometry of a particular CRT and the A.sub.2 acceleration potential. The maximum amount of deflection required is proportional to; the sine of the half angle, to the square root of the anode voltage, and roughly to the neck diameter squared. While the deflection waveforms are basically sawtooth type waveforms, it has been found that when the electron beam deflection center and tube face center of curvature have different radii, the displayed image is distorted. Thus, a correction factor ("S correction") to compensate for the difference in the relative centers of curvature is generally utilized. In addition, the anode and grid biasing voltages vary from CRT to CRT.
It would be desirable if standardized electronic assemblies were available that could be interchanged between units utilizing various size CRTs. Thus, manufacturing procedures could be simplified, and the number of different spare parts required in stock would be minimized. More importantly, Command Control Communication, C3, installations such as NORAD, SAC, etc., which use CRT's of various sizes, could more easily logistically support high priority displays with a minimum of spare electronic modules. However, as noted above, CRT's of different screen size and, for example, their attendant deflection coils require varying circuit parameters. Installations that so utilize displays typically operate at more than one of the scan rates delineated in RS-170 and RS-343. Automatic line rate ranging systems are known in the art, operable to lock in on any received line rate within a wide range of frequencies (generally the range of line rates prescribed by RS-343). Such systems, however, are relatively complex and expensive. Further, the sophistication of the system tends to reduce reliability.